AT THE HEIGHT of the Cold War, during the winter of 1980, FBI agents recorded a phone call in which a man arranged a secret meeting with the Soviet embassy in Washington, D.C. On the day of his appointment, however, agents were unable to catch sight of the man entering the embassy. At the time, they had no way to put a name to the caller from just the sound of his voice, so the spy remained anonymous. Over the next five years, he sold details about several secret U.S. programs to the USSR.

It wasn't until 1985 that the FBI, thanks to intelligence provided by a Russian defector, was able to establish the caller as Ronald Pelton, a former analyst at the National Security Agency. The next year, Pelton was convicted of espionage.

Today, FBI and NSA agents would have identified Pelton within seconds of his first call to the Soviets. A classified NSA memo from January 2006 describes NSA analysts using a "technology that identifies people by the sound of their voices" to successfully match old audio files of Pelton to one another. "Had such technologies been available twenty years ago," the memo stated, "early detection and apprehension could have been possible, reducing the considerable damage Pelton did to national security."